The present invention relates to an electric motor which is provided with a thermal safety feature.
Electric motors of the above mentioned general type are known in the art. One of such electric motors is disclosed for example in U.S. Pat. No. 5,089,735. The known electric motor has a rotor with a rotor winding. The known rotor is provided with a brush for a current transmission from a stationary current terminal of the electric motor to a rotatably supported rotor winding. The brush is received movably in a case and is spring-loaded against a collector of the rotor. The known electric motor is also a collector motor. The present invention is however not limited to collector motors, but instead can be used for example for sliding ring motors or in general for electric motors in which a current transmission is performed to the rotor winding through a brush or a similar sliding body.
In order to provide the protection from thermal overloading, the known electric motors have a thermal safety means. The thermal safety means include a bimetal-interruption contact which is arranged closely on the brush of the electric motor. During heating of the brush and thereby the bimetal-interruption contact, the bimetal interruption contact opens by a switch-off temperature. Thereby the current supply to the rotor winding is interrupted, and a further thermal heating of the electric motor is prevented.
Another electric motor provided with a thermal safety feature is disclosed for example in the European patent document EP-A 171 446. This electric motor has metal spring tongs composed of a shape memory elements, on which the brushes of the electric motor are mounted. The spring tongs press the brushes against a collector of the electric motor. The shape memory elements change their shape abruptly during exceeding of a threshold temperature. The spring tongs composed of the shape memory elements in the known electric motors are shaped and arranged so that in the event of exceeding the threshold temperature, the brushes are lifted from the collector and thereby the current supply to the rotor winding is interrupted.